Voice-frequency key dialling facility



O. BAADE ETAL VOICE-FREQUENCY KEY DIALLING FACILITY 5! $TAT/ON CIRCUITTONE GENERATOR Filed Dec. 8, 1965 TRANSFORMER POWER UNIT Ton E7 SPEECHIMMUNE VOICE- FREQUENCY RELEI VEQ POWER UNIT TRANSFOIQMER 'TUNED VOICE EQUENC Y REC 8/ V5? STANDARD VOICE I r 52 QEC dE United States Patent3,482,051 VOICE-FREQUENCY KEY DIALLING FACILITY Otto Baade, Ditzingen,and Lorenz Gasser, Stuttgart, Germany, assignors to InternationalStandard Electric Corporation, New York, N.Y., a corporation of DelawareFiled Dec. 8, 1965, Ser. No. 512,344 Claims priority, applicationGermany, Dec. 15, 1964,

1,232,213 Int. Cl. H04m 3/42 US. Cl. 179-16 6 Claims ABSTRACT OF THEDISCLOSURE Station signaling with speech immunity and noise exclusionare achieved in a system that can use a speechimmune VF receiver and astandard VF receiver both online at the same time at the receive end.The station circuit alternately presents pulsed-DC key signals andcodepair VF signals. The speech-immune receiver can accept the VPsignals while ignoring the pulsed DC; the standard receiver can receivethe VP signals if activated by a tuned VF receiver responsive to thepulsed DC.

The invention relates to voice-frequency key dialling and particularlyto the transmission of dial information items from a subscriber stationto an exchange.

A number of voice-frequency key dialling methods are known which can bedivided into two groups. The main problem of all these methods is toprovide for so-called speech immunity, i.e. provide measures to assurethat no signals are accurately imitated by the voice frequencies. Basedon the measures selected, the methods can be subdivided into two groups.The first group of methods provides a signal code at the voicefrequencies having characteristics which distinguish it from speechwithin the voice-frequency band and which can be evaluated via speechimmune voice-frequency receivers. In the second group of methods akey-signal, preferably a D. C. signal, is added to each code signalthrough which signal standard voice-frequency receivers can be set readyfor operation.

When weighing the foregoing two groups of methods against each other themethods employing the voice-frequency safe signal code andspeech-immunity voice frequency receivers are preferred where a highsignalling speed is desired and where lines are inserted into thetransmission path, terminating on either end by transformers orcapacitors, which cannot transmit D. C. key signals without extrafacilities. The disadvantge of these methods is that the receivingdevices are more expensive, and in the presence of noise-interference onthe line, they can be blocked to signalling.

The second group of methods employs standard voice frequency receiversand so-called key signals and do not reach a high signalling speed,because, prior to evaluation of the voice frequency code signal, the keysignal must be evaluated and the standard voice frequency receiverconnected thereupon. Moreover, as already indicated, the use of keysignals ordinarily requires the addition of means for transmitting DCsignals at lines terminating on either end by transformers orcapacitors. The big advantage of these methods is, however, that anabsolute safety against interfering speech can be obtained and notroubles are to be feared, even if using the methods on linessusceptible to interfering noise.

It is an object of the invention to provide a voice frequency keydialling system according to the method which provides at a transmissionstation for the operation of either speech immunity voice frequencyreceivers or the operation of standard voice frequency receivers (whichice cannot distinguish transmitted tone from similar voice signals)which are connected for the key signal via the receiver on the receivingend. It is thus an object to combine advantages of both groups ofmethods into operating systems.

The voice-frequency key dialling method, particularly for thetransmission of dial information items between subscriber stations andthe exchange system is characterized, according to the invention, inthat the code signals are formed by a pulsed-DC key-signal (eg, a loopinterruption for a short time) and a speech-immunity signal frequencycombination (e.g. tones taken two-at-atime). Evaluation of the combinedsignal may be performed either by speech immunity voice frequencyreceivers which ignore the key signal, or by standard voice-frequencyreceivers which are switched ready-foroperation by said key signal. Theselection and use of one or the other receiving facility can be madedepending upon the conditions expected to prevail. Thus, for any givenreceiver safe and optimum transmission of information can be assured byuse of the appropriate one of the alternative receiver systems.

A particularly advantageous embodiment of the invention is characterizedin this that, on the receiving end, when using standard voice-frequencyreceivers, the key signal is marked by balancing amplitudes formed byshort-time interruptions of the DC signals, e.g. occurring at a lineinterruption, and that said balancing amplitudes are received by asuitably resigned and inductively connected voice frequency receiver. Insuch a case the key signal can be transmitted via lines, terminating oneither end by transformers or capacitors. The short-time loopinterruption or loop current reduction is produced, according to theinvention, by the switch-over of a contact, particularly a ball-typearmature contact with separate breakand make-contact. Thespeech-immunity signal frequency combinations of the code signals areformed in the two-group code 2 x l-out-of-4 (formed by two tones eachtaken from a group of four tones), according to a particularlyadvantageous design.

The invention is now in detail explained with the aid of theaccompanying drawing which shows, in principle, the method and bothreceiving facilities usable therewith.

The part of the drawing to the left of the dash-dot line 2 shows thetransmitting end, i.e. the subscriber station of a telephone exchange,schematically. The station circuit St is connected, after the handsethas been lifted, with the power unit Sp of the exchange via the line L,from which unit the station receives the feeding current. Fortransmitting dial information items the keys T of the dialling key setare actuated in compliance with the telephone number of the subscribercalled. As indicated at the transmitter TonG via a key contact T thecorresponding voicefrequency code signal is set. Using a two-group code2 x 1- out-of-4, a two-frequency generator is provided which furnishes,after actuation of the key, the associated code signal. At each pressingof the key the switch-over contact u is actuated, too, whereby thestation St is switched off and the transmitter TonG switched on. It isthereby assured, that, during the emission of a voice-frequency codesignal, speech frequencies produced by the microphone of the station Stcannot influence the signalling. The switch-over period of the contact uprecedes, as so-called key signal, the voice frequency code signal. Thisswitchover period can be made independent of the duration of pressingthe key and independent of the speed with which the keys are actuated.An armature contact may be provided having a separate breakandmake-contact and being controlled by a permanent magnet, mounted to amechanical coupling element. During the switch-over period of contact uthe loop current is interrupted or reduced.

These composed code signals can now be evaluated in two different ways.

The right portion of the drawing schematically shows both possibilities;however, it is expected that only one of the illustrated receivers willusually be connected to the line at any given location and any giventime, though both might be used, of course, if desired. The top partshows on the receiving end, besides the unit Sp, 2. speech-immune voicefrequency receiver TonEl, connected to the line. The type of connectionmakes no difference for this receiving facility, because thevoice-frequency portion of the code signal is immune against speech, andthe key signal, accompanying the code signal, can be neglected whenevaluating said code signal. The transmission path can be left in anarbitrary manner via lines, terminating on either end by transformers orcapacitors, and also via other routes.

The lower part shows the second receiving facility. Since only standardvoice frequency receivers TonEZ are used the speech immunity must beobtained via the key signal. To this end a receiver SE is provided forthe key signal which is switched on only when the standard voicefrequency receiver TonEZ responds. The receiver SE can be designed in anarbitrary manner, in order to evaluate the loop interruption or the loopcurrent reduction. Most of the methods known use suitable direct currentevaluators which supervise the potentials on the line wires. Aparticularly advantageous arrangement is obtained, if the balancingamplitudes, occurring at a defined, constant line interruption, and thethere-to connected receiving facility are evaluated. Then acorrespondingly tuned voice frequency receiver SE is inductivelyconnected with the line, as indicated schematically by the transformerUe. The capacitor C performs the required disconnection of directcurrent.

What is claimed is:

1. A voice-frequency key dialling system for the transmission of dialinformation between a calling subscriber station and exchange where theexchange includes as components a choice between a speech immunevoice-frequency receiver, and a standard voice frequency receivercoupled with a tuned voice frequency receiver, said system comprising:

a power unit,

a calling subscriber station including a station circuit and a tonegenerator,

a line having transmit and receive ends,

means for alternately connecting said station circuit and said tonegenerator to the transmit end of said line to alternately transmitpulsed-DC. key-signals and coded voice frequency signal combinationsover said line,

means connecting said power unit to said receive end of said line,

the receive end of said line including as components a choice between aspeech-immune voice frequency receiver and a standard voice frequencyreceiver coupled with a tuned voice frequency receiver, where a speechimmune voice frequency receiver will respond to said signal frequencycombination and a tuned voice frequency receiver will respond to thebalancing amplitudes of the pulsed-DC. key signals and switch on saidstandard voice frequency receiver.

2. A voice frequency key dialling system as claimed in claim 1 in whichsaid tone generator includes means for generating frequency combinationsformed as tones taken tWo-at-a-time from two groups of four tones.

3. A voice frequency key dialling system as claimed in claim 1, in whichthe standard voice frequency receiver at the receive end of said linesenses the key signal for balancing amplitudes occurring at a lineinterruption.

4. A voice frequency key dialling system as claimed in claim 3, in whichany tuned voice frequency receiver is inductively coupled to receive thekey signal.

5. A voice frequency key dialling system as claimed in claim 1, in whichthe means for transmitting a key signal includes means for generating aloop current reduction to form said key signal.

6. A voice frequency key dialling system as claimed in claim 5, in whichthe means for generating the loop current reduction includes means formaking and breaking contact through said line to said power unit.

References Cited UNITED STATES PATENTS 3,200,205 8/1965 Herter.2,332,912 10/1943 Hecht et a1. 179-16 3,319,011 5/1967 Maurushat 17916KATHLEEN H. CLAFFY, Primary Examiner T. W. BROWN, Assistant Examiner US.Cl. X.R. 179-84

